The crankshaft sensors are used in a motor vehicle to determine the position of the crankshaft, the speed of rotation and the direction of rotation of the engine. Used in combination with camshaft sensors, they determine the position of the different cylinders in the combustion cycle of the engine (that is to say determining, for each cylinder, whether it is in intake phase, in compression phase, in explosion phase or in exhaust phase) and make it possible to best manage the operation of the engine, by the optimal setting of the ignition advance or of the fuel injection instant.
These crankshaft sensors comprise a magnetic field generator (example: a permanent magnet), a magnetic field detection means (Hall-effect cell, magneto-resistive MR cell, giant magneto-resistive GMR cell, for example) and an electronic circuit for processing the signal received by the magnetic field detection means. These sensors, called active sensors, deliver a digital signal to processing means.
The magnetic field generator can also be a target, consisting of a magnetic material, exhibiting north and south pole alternations. In this case, the sensor may or may not incorporate a permanent magnet depending on the detection means used. Hereinbelow, the north and south poles are compared to the teeth and to the hollows of a mechanical target.
As is known and as illustrated in FIG. 1, a crankshaft sensor 10 is associated with a target 14 secured to a crankshaft 16. This target 14 takes the form of a disk 15 whose periphery is toothed. Between teeth T1, T2, T3, that are substantially identical, there is a space (hollow) C1, C2, C3. The target 14 is distinguished by the presence of a hollow Ce of greater length, more commonly called “missing tooth”, positioned precisely at a certain angle relative to the angular position of the engine. According to the embodiment described and represented in FIG. 1, a crankshaft sensor 10 comprises, as is known, a ferromagnetic element 11 and a magnetic field detection means 12 (for example a Hall-effect cell). This sensor 10 delivers a digital signal to one of the processing means 13.
The operation of such a sensor assembly 10 and the associated target 14 is described hereinbelow.
When the target 14 is driven in rotation in one direction (arrow AV FIG. 1) by the crankshaft 16, the sensor 10 perceives a series of variations of the magnetic field representative of the tooth or teeth T1, T2, T3 passing in front of it and of their spacing C1, C2, C3, Ce (see FIG. 2). The signal that is thus obtained is represented in FIG. 3.
FIG. 3 shows, according to the prior art, the signal B of the magnetic field delivered by the sensor 10 as a function of the angle of rotation θ of the crankshaft 16 (or of the target 14), and the threshold S1 of detection of the rising edge and of the falling edge of the first tooth T1. FIG. 2 represents the position of the teeth T1, T2, . . . Ti and of the hollows C1, C2 . . . Ci of the target 14 relative to the signal B of the magnetic field of FIG. 3.
As illustrated in FIG. 3, to determine the position of the crankshaft, the signal B representing the variations of the magnetic field perceived by the sensor 10 of the crankshaft 16 is observed during a revolution of the target 14, that is to say according to an angle of rotation θ of the target 14. This signal exhibits a series of sinusoids D1, D2 . . . Di each corresponding to the variation of the magnetic field measured by the sensor 10 when a tooth T1, T2 . . . Ti (see FIG. 3) followed by a hollow C1, C2 . . . Ci passes in front of said sensor 10. By counting the number of sinusoids D1, D2 . . . Di, by measuring the duration of each of them, the spacing between each sinusoid D1 D2 . . . Di, and by detecting the missing tooth (the spacing due to the missing tooth Ce being longer), it is possible to determine the speed of rotation of the engine, the direction of rotation of the engine and the angular position of the crankshaft.
As illustrated in FIG. 3, the signal B exhibits a minimum BMIN1 and a maximum BMAX1. The detection of the passage of the teeth T1, T2 . . . Ti and of the hollows C1, C2 . . . Ci of the target 14 is made by the detection of the passage of the signal B above (respectively below) a detection threshold S1 placed between the minimum BMIN1 and the maximum BMAX1, for example equal to S1=k1*(BMAX1−BMIN1), k1 being a constant, for example equal to 0.50.
For explanatory purposes, the signal B illustrated in FIG. 3 comprises a single minimum BMIN1 and a single maximum BMAX1. In reality, the signal B exhibits a plurality of minimums BMINi and a plurality of maximums BMAXi and the detection threshold S1 is continually adapted according to the minimums and maximums in order to be always equal to S1=k1*(BMAXi−BMINi). This method for adapting the detection threshold S1 is known to those skilled in the art, see patent application FR 2 985 035 A1 filed by the applicant which describes the same method for adapting the detection threshold but applied to a camshaft sensor.
The magnetic field variations perceived by the sensor 10 (signal B in FIG. 3) are processed by the sensor 10, and the latter sends, to the processing means 13, a signal S of voltage 0-5 V (see FIG. 4) comprising pulses I from a high state (5 V) to a low state (0 V). Each pulse I is representative of the passage of the middle of a tooth T1, T2 . . . Ti in front of the sensor 10, that is to say the crossing of the detection threshold S1 from the maximum value BMAX1 of the signal B to the minimum value BMIN1.
When the target 14 revolves in “forward drive mode” (see arrow AV), the pulse I has a first predetermined duration t1, for example 45 μs. When the target 14 revolves in “reverse drive mode” (arrow AR), the pulse I has a second predetermined duration t2, for example 90 μs.
The detection of the direction of rotation of the target 14 “forward” or “reverse” is known to those skilled in the art, and will not be described in detail here. In effect, the sensor 10 generates a second signal, called direction signal (not represented), whose phase difference with respect to the signal S indicates the direction of rotation of the target 14.
The processing means 13 receive the signal S and the direction signal and then determine the speed of rotation, the direction of rotation of the crankshaft, that is to say of the engine.
Upon an electrical malfunction of the sensor 10, that is to say a short circuit to ground, or an open circuit, the signal S at the output of the sensor 10 takes either the value 0 V or the value 5 V (respectively).
However, such a sensor 10 does not provide information to the processing means 13 regarding any malfunction of the system 20 for measuring speed and direction of rotation of the crankshaft.
Malfunction of the system should be understood to mean:                an air gap defect (also called air gap distance defect) or defect of alignment between the sensor 10 and the target 14 or misaligned therewith, the sensor 10 is in this case too far away from the target 14, and the variations of the magnetic field perceived by the sensor 10 are too weak to accurately determine the speed and the direction of rotation of the crankshaft,        a “radial runout” or defect of eccentricity of the target 14, the target is then fixed onto the crankshaft in such a way that when it revolves, it oscillates in its plane, or it is not centered on the axis of the crankshaft, in these two cases, the variations of the magnetic field over a revolution of the target 14 are significant and affect the accuracy of the signal S. As for the preceding defects, the determination of the speed and of the direction of rotation of the crankshaft can prove impossible.        